Ultrasonic flowmeters are used often in process- and automation-technology. They permit in simple manner contactless determination of volume flow rate in a pipeline.
Known ultrasonic flowmeters operate on the basis of either the Doppler principle or the travel time difference principle.
In the travel time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid.
For this, ultrasonic pulses are transmitted both in the direction of flow and opposite thereto. From the travel time difference, one determines the flow velocity, and, with that, knowing the diameter of the pipe section, the volume flow rate.
In the Doppler principle, ultrasonic waves of a known frequency are coupled into the liquid, and the ultrasonic waves reflected from the liquid are evaluated. Using the frequency shift between the waves coupled in and the reflected waves leads likewise to the flow velocity of the liquid.
Reflections in the liquid occur, however, only when small air bubbles or impurities are present therein, so that this principle finds application primarily in the case of contaminated liquids.
The ultrasonic waves are produced and received in a measuring head. The particular measuring head is fixed on the wall of the pipe section of concern. Recently, clamp-on ultrasonic measurement systems have become available. In these systems, the measuring head is only pressed with a clamp on the pipe wall. Such systems are described e.g. in EP-B 686,255, and the U.S. Pat. Nos. 4,484,478 and 4,598,593.
The essential elements of a measuring head are the measuring head housing, a piezoelement with connection unit, and an ultrasonic coupling element made of plastic. The ultrasonic waves are produced in the piezoelement and guided to the pipe wall through the ultrasonic coupling element. From there, they are conducted into the liquid.
In a known measuring head, the ultrasonic coupling element extends somewhat beyond the measuring head housing, so that only the ultrasonic coupling element contacts the measurement pipe. Due to the low heat conductivity of the ultrasonic coupling element, only a small heat transfer from the pipe wall to the measuring head is possible. For this reason, temperature gradients can arise in the measuring head, gradients which negatively affect the measurement accuracy.
An additional disadvantage of known measuring heads is that measuring head housing and ultrasonic coupling element are adhesively bonded. This requires very small tolerances in the manufacture of the measuring head housing. Cast parts, which have greater tolerances, therefore require expensive finishing operations.